The present invention relates to a method for forming a connecting sleeve having an internal groove at the end section of a tube of thermoplastic material in which the end section is heated to the deformation temperature, is then, in order to increase its wall thickness, axially compressed or upset in a heatable upsetting device which defines an annular gap until the annular gap is filled, is thereafter shaped to form the connecting sleeve while being axially displaced in a mold, and is finally cooled.
Method and apparatus for implementing such sleeve forming operations are known in the art. Reference is made, for example, to German Pat. No. 1,257,413, which discloses an apparatus for providing hollow plastic bodies with sleeve ends, annular grooves and thickened walls. The tube ends worked with this apparatus are preheated for deformation, for example, in so-called annular gap furnaces (annular heating devices) and are then given their final shaping in a molding device. During this final shaping, an upsetting force may be applied which compensates for the weakened wall portions developed during the shaping by pushing in heated tube wall material until the original wall thickness is regained. See also, U.S. Pat. Nos. 3,553,780 and 3,557,278.
German Auslegeschrift No. 2,319,398 and corresponding U.S. Pat. No. 3,899,565 further disclose a method, as well as an apparatus containing an inner mandrel and a hollow outer mold which define an annular gap, for forming a sleeve having an annular groove at the end section of the tube of thermoplastic material, wherein a thickening of the end section takes place in the form of preliminary upsetting at a temperature above the softening (deformation) temperature. Preliminary upsetting of the end section is here progressive from a region remote from the frontal face of the end section of the tube toward the frontal face. The preliminary upsetting process is continued until the annular gap existing between the inner mandrel and the outer mold has been filled with the softened plastic material of the tube wall and of the thickened wall in this region. The filling of the annular gap takes place from the entrance opening of the mold, that is, where the tube end section enters the mold, toward the bottom of the annular gap in the interior of the mold.
After the preliminary upsetting in the prior art process disclosed in German Auslegeschrift No. 2,319,398 and the corresponding U.S. Pat. No. 3,899,565, the sleeve is formed by displacing the upset end section onto a widened section of the mandrel which is at a lower temperature than the deformation temperature of the plastic material and which acts as a sleeve forming device. This produces a certain stabilization of the peripheral regions of the sleeve surface already during the sleeve forming process, which stabilization is reinforced by the fact that the inner wall of the outer mold is also kept at a temperature below the deformation temperature of the plastic during the sleeve forming process.
An annular inner (peripheral) groove is then formed in the sleeve by pressing a spreading element of the sleeve forming device into the end section with its partially stabilized sleeve. For this purpose, the sleeve forming device is provided with a groove-forming member in the form of a rubber body which can be pressed from the inner side of the mandrel by a cone shaped expanding part.
In the known process disclosed in German Auslegeschrift No. 2,319,398 and the corresponding U.S. Pat. No. 3,899,565, the shaping of the connecting sleeve from the thickened upset end section is thus performed by axial displacement of the end section on the widened portion of the inner mandrel and subsequent pressing to form the annular circumferential groove by means of known spreading members. It is here important that the thickening process, as well as the subsequent shaping process for forming the connecting sleeve, are performed by axial and radial displacement in one and the same apparatus. The thickening of the wall must be completed before the shaping process for forming the connecting sleeve begins. During the sleeve shaping process itself--as a result of the thickening tube end being pushed onto the widened portion of the inner mandrel--there may occur a solidification of the sleeve walls, but pressing in the sleeve to form the annular goove further weakens the wall cross section in the sleeve region without this drawback being able to be overcome by further upsetting.
Finally, German Offenlegungsschrift No. 2,805,518 discloses an apparatus for shaping seal receptacles in sleeve connections for plastic tubes. This publication discusses the problems encountered in fixing fit dimensions in the interior of the sleeve and, in particular, for the sleeve member which is to receive the seal. In addition to a special configuration of the mold ring for producing the annular groove, this publication neither makes particular mention of the problem of preliminary upsetting nor of any possible difficulties encountered in the shaping of the cylindrical sleeve portion.
The processes and apparatus known in the art are able to handle without difficulty tubes of plastic materials having an amorphous structure, such as, for example hard PVC, since these plastics have the property of softening in a temperature range between 90.degree. and 140.degree. C. and can be easily worked continuously while the temperature is increasing with a uniformly progressive heat supply. Between the stated temperature regions, the material is thermoplastic and forces for its deformation are dependent on the respective temperature of the tube.
The prior art methods and apparatus, however, are unable to correspondingly change the shape of tubes made of crystalline or partially crystalline thermoplastic materials such as polyethelene, polypropylene, and the like.
The crystalline components of these materials are converted to the amorphous or partially amorphous state by heating, and this conversion results in an increase in volume, i.e. swelling of the material. Maximum swelling is realized when the crystalline melting temperature is reached or exceeded. Problems encountered during heating of tubes made of crystalline or partially crystalline plastics arise, for example, because the reduction of the crystalline components may be nonuniform over the cross section of the tube due to differences in tube wall thickness and due to the manner of heating of the tube. This produces irregular swelling which is further promoted by the release of internal stresses in the tube. The crystalline or partially crystalline thermoplastic materials, in contrast to the amorphous plastics, exhibit no continuous softening curves during heating, so that irregular swellings result in the radial and axial directions of the tube end section. Such swollen tube ends are useless for further processing.
Although U.S. Pat. No. 3,899,565 discloses heating and upsetting of a tube made of a crystalline plastic, e.g. polyethylene, beginning in the region of the end section, the upsetting is done in one process step, and the subsequent shaping of the sleeve and of the circumferential groove weakens the upset tube wall material again in these regions, so that, particularly in the susceptible region of the circumferential groove, the widening of the circumference of the end section as a result of the sleeve shaping results in a considerable weakening of the tube end section walls which may easily lead to a break in this region of the tube which is particularly heavily stressed.